Cable seal

ABSTRACT

A cable seal and method for sealing a cable termination is provided for sealing the passage of a cable into a cable-receiving structure. The seal includes a first plurality of o-rings disposed along a length of the cable adjacent the cable-receiving structure and a first length of heat-shrinkable tube which fits over the cable and o-rings for compressive engagement with the cable and o-rings to prevent moisture penetration between an inner surface of the first tube and the outer surface of the cable. A second plurality of o-rings is disposed over the first heat-shrinkable tube adjacent the first plurality of o-rings. A second length of heat-shrinkable tube fits over the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and a portion of the cable-receiving structure to prevent moisture penetration between an inner surface of the second tube and an outer surface of the cable-receiving structure and between the inner surface of the second tube and the outer surface of the cable.

BACKGROUND

This invention relates generally to a cable seal, and more particularlyto a cable seal for a pressure vessel assembly for housing electroniccomponents in an underwater environment.

Underwater communication systems provide signal transmission between, orto, land-based positions. A typical system generally comprises a cablefor signal transmission and one or more housings containing electricalcomponents which are spaced along the cable between the land-basedpositions. The signal cable has a core which may include electricalconductors, fiber optic cable, or other signal transmission elementssurrounded by a protective jacket. Metal strength members are disposedabout the core of the cable between the cable core and the outer surfaceof the jacket. The strength members bear the tensile and compressiveloads placed on the cable while in operation. The cable may carryvarious signals, including low voltage signals such as information anddata signals, higher voltage signals for providing electrical power, orother types of signals.

The housings for the electronic components are referred to as pressurevessels. A pressure vessel is typically a cylindrical tube with openends capped by circular bulkheads. The signal cable is terminatedadjacent to each of the bulkheads in a termination assembly. Thetermination assemblies are secured to the bulkheads and providemechanical continuity between the cable ends and the pressure vesselwhile relieving the stress on the signal transmission elements of thecable. The signal transmission elements pass into the pressure vesselthrough seals in the bulkheads for connection to the electroniccomponents in the pressure vessel.

The pressure vessel assembly must protect the electronic components,signal cable and their connections from exposure to water at depths ofup to 20,000 feet and pressures of up to 10,000 pounds per square inchfor periods of up to 25 years. This harsh environment contributes toproblems related to the performance reliability and product life ofpressure vessel assemblies.

Conventional solutions to the problems of designing reliable and durablepressure vessel assemblies are plagued by high cost. The presentpractice is to use pressure vessels formed from beryllium-copper ortitanium with polyethylene or gland cable seals andpolyethylene-overmolded cable termination assemblies. Beryllium-copperor titanium is used for the pressure vessel because of their excellentresistance to corrosion in underwater applications. However, thismaterial is very expensive, and machining is difficult. The polyehtyleneor gland cable seals are expensive, consist of numerous parts, and aredifficult and time-consuming to install. In addition to the seals, awater block of some sort must be used to prevent water ingress in theevent of a cable cut.

The termination assemblies are overmolded with polyethylene to preventwater from accessing the internal portions of the signal cable andpressure vessel. In the overmolding process, high density polyethyleneis molded around the cable termination assembly thereby sealing theareas between the outer surface of the cable jacket and the terminationassembly. In some cases, portions of the cable and the pressure vesselare also overmolded with polyethylene. However, polyethylene overmoldingis not cost effective in most applications because the required moldingequipment is expensive and the process time consuming therebyrestricting production rates. Moreover, the overmolding is a difficultprocess, requiring a high operator skill level and has yield andreliability problems.

For the foregoing reasons, there is a need for a reliable, long life,low cost pressure vessel assembly for housing electrical components inunderwater communication systems. The new pressure vessel assembly mustbe capable of withstanding deep underwater pressures for many years.Seals for the passage of the signal cable transmission elements into thepressure vessel should be easy to install and effectively preventmoisture penetration. The termination assembly sealing process should befast and simple to perform, requiring minimal operator skill level.Moreover, the components for sealing the cable termination assemblyshould be adaptable to seal various cable termination assembly types.

SUMMARY

Therefore, it is an object of the present invention to provide a lowcost underwater pressure vessel assembly which is readily andeconomically produced for use in underwater communications systems.

A further object of the present invention is to provide moistureprotection to the electronic components in the pressure vessel. Arelated object is to provide a simple, effective seal for the signaltransmission elements passage into the pressure vessel.

A still further object of the present invention is to provide a seal andmethod for readily and simply sealing a cable termination assembly foruse in connecting signal cable to the pressure vessel. The terminationassembly seal and method should allow for adaptability to various cabletypes.

Another object of the present invention is to provide a pressure vesselassembly which is reliable and durable enough for an extended usefullife submerged in the underwater environment.

According to the present invention, a pressure vessel is provided forhousing electronic components in an underwater environment andpermitting connection of the components to signal transmission elementsof a signal cable. The pressure vessel comprises a hollow steel shelldefining an interior chamber adapted to house the electronic components.A layer of thermal-sprayed aluminum covers the shell. The shell has anopening adapted to pass the signal transmission elements into theinterior chamber. A seal adapted to sealingly surround the transmissionelements is disposed in the opening in the shell so that an outerperipheral surface of the seal contacts an inner peripheral surface ofthe opening in the shell for preventing moisture penetration into theinterior chamber. The seal may be formed of epoxy and the outerperipheral surface have a plurality of compressible o-rings disposed inaxially-spaced circumferential grooves for contacting the innerperipheral surface of the opening in the shell.

Also according to the present invention, an underwater telecommunicationsystem is provided comprising electronic components and a hollow steelshell for housing the electronic components. The shell is covered by alayer of thermal-sprayed aluminum. A signal cable is mechanicallyconnected to the shell in watertight relation. The signal cable includesat least one transmission element and the shell has an opening forpassing the transmission element into the interior chamber forconnecting the transmission element to the electronic components forsignal transmission. A seal surrounding the transmission element isdisposed in the opening in the shell for preventing moisture penetrationinto the shell.

According to another aspect of the present invention, a cable seal isprovided for sealing the passage of a cable into a cable-receivingstructure. The seal comprises a first plurality of o-rings disposedalong a length of the cable adjacent the cable-receiving structure and afirst length of heat-shrinkable tube which fits over the cable ando-rings for compressive engagement with the cable and o-rings when thefirst tube is heated to prevent moisture penetration between an innersurface of the first tube and the outer surface of the cable. A secondplurality of o-rings is disposed over the first heat-shrinkable tubeadjacent the first plurality of o-rings. A second length ofheat-shrinkable tube fits over the second plurality of o-rings, thefirst heat-shrinkable tube, the first plurality of o-rings, the cableand a portion of the cable-receiving structure for compressiveengagement with the second plurality of o-rings, the firstheat-shrinkable tube, the first plurality of o-rings, the cable and theportion of the cable-receiving structure when heated to prevent moisturepenetration between an inner surface of the second tube and an outersurface of the cable-receiving structure and between the inner surfaceof the second tube and the outer surface of the cable.

According to a still further aspect of the present invention, a sealedcable end assembly comprises a structure having a through passage forreceiving a cable end and means for preventing relative axial movementof the structure and cable. A first plurality of o-rings is disposedalong a length of the cable adjacent the cable-receiving structure. Afirst length of heat-shrinkable tube is positioned around the cable ando-rings for compressive engagement with the cable and o-rings when thefirst tube is heated to prevent moisture penetration between an innersurface of the first tube and the outer surface of the cable. A secondplurality of o-rings is disposed over the first heat-shrinkable tube,the second plurality of o-rings positioned adjacent the first pluralityof o-rings. A second length of heat-shrinkable tube fits over the secondplurality of o-rings, the first heat-shrinkable tube, the firstplurality of o-rings, the cable and a portion of the cable-receivingstructure for compressive engagement with the second plurality ofo-rings, the first heat-shrinkable tube, the first plurality of o-rings,the cable and the portion of the cable-receiving structure when heatedto prevent moisture penetration between an inner surface of the secondtube and an outer surface of the cable-receiving structure and betweenthe inner surface of the second tube and the outer surface of the cable.

Also according to the present invention, a method is provided forsealing a cable termination including a cable end positioned in anopening of a cable-receiving structure for preventing relative axialmovement of the cable end and structure. The sealing method comprisesdisposing a first plurality of o-rings along a length of the cableadjacent the cable-receiving structure and positioning a first length ofheat-shrinkable tube around the cable and o-rings. The firstheat-shrinkable tube is heated causing the tube to compressively engagethe cable and o-rings to prevent moisture penetration between an innersurface of the first tube and the outer surface of the cable. A secondplurality of o-rings is disposed around the first heat-shrinkable tubeadjacent the first plurality of o-rings and a second length ofheat-shrinkable tube is positioned around the second plurality ofo-rings, the first heat-shrinkable tube, the first plurality of o-rings,the cable and a portion of the cable-receiving structure. The secondheat-shrinkable tube is then heated causing the tube to compressivelyengage the second plurality of o-rings, the first heat-shrinkable tube,the first plurality of o-rings, the cable and the portion of thecable-receiving structure to prevent moisture penetration between aninner surface of the second tube and an outer surface of the portion ofthe structure and between a portion of the inner surface of the secondtube and the outer surface of the cable.

The pressure vessel assembly of the present invention provides a lowcost, structurally robust, reliable, and durable device for use inunderwater communication systems. The thermal-sprayed aluminum coatingof the steel pressure vessel provides a housing for the electroniccomponents of the system which is resistant to corrosion, particularlygalvanic corrosion in seawater. The two-layers of heat-shrinkable tubesover sets of o-rings function as multiple redundant seals for the cabletermination assembly. The components of the pressure vessel assembly ofthe present invention are low cost and require minimal assemblyexpertise and time.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, referenceshould now be had to the embodiments shown in the accompanying drawingsand described below. In the drawings:

FIG. 1 is a perspective view of a pressure vessel assembly according tothe present invention showing a portion of the cables extending fromeach end;

FIG. 2 is an exploded view of an electronics pressure vessel assembly asshown in FIG.1;

FIG. 3 is a close-up exploded view of a bulkhead assembly for use withthe electronics pressure vessel assembly as shown in FIG. 2;

FIG. 4 is a close up view of a cable seal for use in the presentinvention;

FIG. 5 is a partial cross-section of an electronics pressure vesselassembly taken along line 5—5 of FIG. 2;

FIG. 6 is a perspective view of a first plurality of o-rings in placealong a cable leading into a cable termination assembly;

FIG. 7 shows a first heat-shrink tube in place around the first set ofo-rings shown in FIG. 6;

FIG. 8 shows a second set of o-rings in place around the shrunken firstheat shrink tube around a cable leading into a termination assembly asshown in FIG. 7;

FIG. 9. shows a second heat-shrink tube around the second set of o-ringsshown in FIG. 8;

FIG. 10 shows the shrunken second heat-shrink tube around a cableleading into a termination assembly as shown in FIG. 9;

FIG. 11 shows a third heat-shrink tube positioned around a cabletermination housing as shown in FIG. 10;

FIG. 12 shows the shrunken third heat-shrink tube around the cabletermination housing as shown in FIG. 11;

FIG. 13 is a side elevational view partially cut-away of the sealedcable termination assembly shown in FIG. 12; and

FIG. 14 is side elevational view partially cut-away of anotherembodiment of a sealed cable and cable termination assembly.

DESCRIPTION

Certain terminology is used herein for convenience only and is not to betaken as a limitation on the invention. For example, words such as“upper”, “lower”, “left”, “right”, “horizontal”, “vertical”, “upward”,“downward”, “clockwise” and “counter-clockwise” merely describe theconfiguration shown in the figures. It is understood that the componentsmay be oriented in any direction in the terminology, therefore, itshould be understood as encompassing such variations unless specifiedotherwise.

Referring now to the drawings wherein like reference numerals designatecorresponding or similar elements throughout the several views, there isshown in FIG. 1 a pressure vessel assembly according to the presentinvention, generally designated at 20, for use in an underwatercommunication system. The pressure vessel assembly includes a pressurevessel 22, a signal cable 24, portions of which are shown extending fromeach end of the pressure vessel assembly 20, and a bend strain relief 26surrounding the signal cable 24 at each end of the pressure vessel 22.

Other components of the pressure vessel assembly 20, are seen in theexploded view of FIG. 2, including bulkheads 28, 30 for sealing the ends32, 34 of the pressure vessel 22 in cooperation with a lock ring 36, andtermination assemblies 38 at each end of the signal cable 24.

The pressure vessel 22 is a hollow, cylindrical shell formed from carbonor stainless steel for housing a card cage assembly 40 (shown in phantomin FIG. 2) supporting card trays carying electronic components of thecommunications system. The pressure vessel 22 has an inner surfacedefining a cylindrical bore having annular seats on each end.

The outer surface pressure vessel 22 is coated with a layer of aluminum44 42 (FIG. 5). Preferably, the aluminum layer 44 is applied using athermal spraying process. Thermal spraying is a process of depositing onsubstrate materials molten or semi-molten materials which solidify andbond to the substrate. The process is also called metallizing and flameor metal spraying. The spray materials may be in the form of wire, rod,or powder. As the materials pass through the spray unit, they are heatedto a molten or semi-molten state and then projected onto the substrate.A coating of thermal-sprayed aluminum (TSA) has been shown to provide a20-year life to steel structures in sea water splash zones. The TSAcoating improves corrosion resistance by providing a “sacrificial”aluminum anode over components susceptible to corrosion in seawater.Thus, in the present invention, the TSA layer 44 will galvanicallyprotect the carbon steel base material of the pressure vessel 22 therebyslowing the corrosion of the pressure vessel to a negligible level.Preferably, a layer of silicone aluminum sealer 46 is applied over theTSA layer 44 to further increase the life of the pressure vessel.

The bulkheads 28, 30 (FIG. 2) form the ends of the pressure vessel 22and are preferably machined from the same grade of carbon steel as thepressure vessel. The bulkheads 28, 30 are generally circular in crosssection with an outwardly extending central cylindrical coaxial portion48. Each bulkhead 28, 30 is secured to the card cage assembly 40.

One of the bulkheads 28 has a cross section which is slightly less thanthe diameter of the inner cylindrical bore of the pressure vessel 22 forallowing insertion of the assembled bulkheads 28, 30 and card cageassembly 40 through one end of the pressure vessel. This smallerbulkhead 28 is externally threaded 50 for receiving the lock ring 36which has an inner thread 52 for engaging the external threads 50 on thebulkhead 28 so that the inner surface of the lock ring contacts the seat(not shown) inside the end of the pressure vessel 22. After thebulkheads 28, 30 and the card cage assembly 40 are inserted into thepressure vessel 22, the bulkheads are welded to the pressure vessel toform an enclosed pressure vessel.

A close-up view of the larger bulkhead 30 is shown in FIG. 3. Theoutward extension 48 includes three portions of varying diameter,including a large externally threaded portion 54, a small intermediateportion 56 having external circumferential grooves, and a distal housing58 having circumferentially-spaced threaded openings 60. The bulkheads28, 30 electrical power lines 66, fiber optic cable 68 are provided withaxial openings 62 which allow them to pass signal transmission elements64 in the signal cable 24, such as telecommunication lines 70, and thelike, into the pressure vessel 22.

According to the present invention, the openings 62 into the pressurevessel 22 are protected against moisture penetration using seals 72. Asseen in FIG. 4, the seal comprises a cylindrical body 74 formed tocorrespond to the size of the outer end of the bulkhead openings 62. Theseal 72 is formed from a thermal setting polymeric material such as anepoxy resin or cross-linked polymer, for example, a cross-linked elasticpolymer. The seals 72 are formed by casting the transmission elements 64in an epoxy base that hardens to provide a strong, non-porous moistureseal around the transmission elements 64. Short lengths of transmissionelements 64 extend out from the ends of the seal 72 for connection tocomponents of system. The formed epoxy seal 72 has three, spaced annulargrooves 76 for holding two radial o-rings 78 and one face-seal o-ring80, respectively. Back-up rings 82 fit in two of the radial seal grooves76 of the epoxy seal -to prevent extrusion of the radial o-rings 78 athigh pressure and also compensate for loose tolerance in the bulkheadopenings 62.

Referring to FIG. 5, the epoxy seals 72 are slid into the outer end ofthe bulkhead openings 62 for introducing the transmission elements 64through the openings and into the pressure vessel 22 where thetransmission elements may be connected to the electrical components.Insertion depth of the seals 72 into the bulkhead 30 is limited by thedepth of a larger diameter outer bore of the openings 62. The seals 72are positively secured within the bores by retaining c-clips 84. so thatthe face seal o-ring 80 engages the bottom of the bore. The radialo-rings 78 seal the periphery of the epoxy seal 72 against the innersurface of the bore to provide a fluid tight seal that prevents anyfluid from reaching the interior of the pressure vessel 22. Thetransmission element 64 ends from the outer end of the seal 72 extendsexternally of the bulkhead 30. A fiber splice tray 86 (FIG. 3) isattached in bulkhead housing 58 using a c-clip 88 when necessary forsplicing fiber optic cable.

A third axial opening 90 in the bulkhead is provided for vacuumevacuation of the pressure vessel 22 and for filling the evacuatedvessel with nitrogen, as is known in the art

Referring now to FIG. 2, cable termination may be accomplished usingstandard cable termination assemblies 38, which are generally of thecone-in-socket or crimp type. In both types, the termination assemblyhousing 92 is mechanically fastened to the signal cable 24 end usingconventional fastening means for securing the metal strength members inthe signal cable 64 to the housing 92. As described above, thetermination assembly 38 functions to pass the signal transmissionelements 64 while assuming the mechanical stress on the signal cable 24.The termination assembly housing 92 holding the terminal end of thesignal cable 24 is designed to prevent the entry of water into theinternal portions of the signal cable. A fiber splice tray 94 isprovided when necessary for splicing of fiber optic cable.

A carbon steel connector sleeve 96is provided for connecting thepressure vessel 22 and termination assembly 38. The ends of the sleeve96 fit tightly over the termination assembly housing 92 and bulkheadhousing 58. The ends of sleeve 96 have circumferentially-spaced holes 98in the periphery which align with corresponding holes 99 in thetermination assembly housing 92 and the holes in the bulkhead housing.The holes in the sleeve 96 receive screws or other fasteners (not shown)for securing the bulkhead housing 58 and the termination assembly 38.When connected, the sleeve 96 houses the optical fiber splice traysassemblies 86, 94 and defines an area where other transmission elements64 are spliced or branched. The sleeve 96 thus becomes a load bearingmember of the pressure vessel assembly 22 to prevent mechanical stressfrom being applied to the transmission elements 64. Moreover, the signalcable 24 is secured relative to the pressure vessel 22 such that theapplication of force to pull the cable from the cable terminationassembly 38 will not be transmitted to connections between the signalcable transmission elements 64 and electronic components in the pressurevessel 22.

The internal area defined by the cable termination assembly 38, bulkheadhousing 58 and sleeve 96 not otherwise occupied is substantially filledwith a water blocking compound, such as a polybutene compound, tosubstantially prevent the entry of moisture into the area and the vessel22. This is accomplished by providing passages 100 (FIG. 5) at thebottom of the screw holes 60 in the bulkhead housing 58 which open intosaid area. During assembly, two opposed screw holes 60 are left open andpolybutene is added to the area through one hole until the polybuteneexits the opposed hole indicating the area is filled.

According to the present invention, a series of heat shrink tubes 102,104, 106 are used to seal the termination assembly 38. Heat-shrink tubesare known and include, for example, polyolefin polymeric materials witha low shrinking temperature such as the polyolefin marketed under thetrade name “Sigmaform” by the Raychem which shrinks completely at a lowtemperature of about 250 degrees Fahrenheit.

After the termination assembly 38 is connected through the sleeve 96 tothe bulkhead housing 58, a length of cable adjacent the terminationassembly housing 92 (FIG. 6) and a portion of the housing 92 are lightlyabraded. The abraded surfaces are then cleaned with industrial gradealcohol. A first set of three o-rings 108 is installed along the cable24 adjacent the housing 92. The first heat-shrink tube 102 is positionedover the o-rings 108 and cable 24 (FIG. 7). The tube 102 is selected tohave a slightly greater inside diameter than the outside diameter of theo-rings 108 so that the tube slips easily over the o-rings. One end ofthe tube 102 seats against the termination assembly housing 92.Preferably, the inside of the tube 102 is coated with an adhesivesealing material which, along with the roughened cable surface, promotesadherence and sealing by the tubing 102. A suitable adhesive isavailable under the designation “S-1030”—“from the Raychem company. Aheat gun (not shown) is used to shrink the diameter of the tube 102(FIG. 8). The shrinking of the tube 102 causes a portion of an innersurface of the heat-shrinkable tube to come into compressive engagementwith the outer jacket of the cable 24 and the o-rings 108 and causes theadhesive to flow providing an excellent water blocking seal even in thepresence of high exterior water pressure. The compressive engagement ofthe tube 102 with the jacket and o-rings 108 also provides a series ofredundant seals between a portion of an inner surface of the heat-shrinktube and the signal cable 24 jacket.

After cooling, the surface of the first heat shrink tubing 102 isabraded to form a rough surface and cleaned with alcohol. As seen inFIG. 8, a second set of three o-rings 110 is installed over the firstheat shrink tubing 102 just behind the first three, already-installedo-rings 108. A second, larger heat-shrinkable tube 104 is positionedover the o-rings 110 and cable 24 over a portion of the terminationassembly housing 92 (FIG. 9). The preferred length of the secondheat-shrink tube 104 is sufficient to cover the first tube 102. Thesecond tube 104 is heated and shrunk down over the cable 24, and O-rings110 and housing 92 (FIG. 10). Preferably, the termination assemblyhousing 92 includes a peripheral flange which is engaged by the shrunkentube 104 for sealing the end of the tube 104 around the housing 92.

As seen in FIG. 11, a third heat-shrinkable tube 106 is then placed overthe sleeve 68 and portions of the bulkhead housing 58 and terminationassembly housing 92 and shrunk into place (FIG. 12). The shrunken tube106 engages the series of grooves provided around the intermediateportion 56 of the axial extension of the bulkhead 30 for sealing thecorresponding end of the tube 106. The third heat-shrink tube seal worksto keep moisture from the area in the sleeve 96 and to retain thepolybutene in the sleeve 96.

The rubber bend strain 26 relief is attached to each bulkhead 28, 30 toprotect the signal cable 24 from being bent at a radius that coulddamage the cable. The bend strain relief 26 (FIG. 2) has a profiledpassage extending therethrough for passing the cable 24. The profiledpassage has a forward bore 112 which is sized to accept the terminationassembly 38. The end of the bend strain relief 26 holds a metal ring 114which is internally threaded to cooperate with the external threads ofthe larger portion 54 of the bulkhead extension 48 to close and seal theend of the pressure vessel assembly 20

Moisture ingress barriers to the electronics within the pressure vessel22 are provided by the heat-shrinkable tubing 102, 104, 106, o-rings108, 110, the water-excluding polybutene compound, and the epoxy seals72. Thus, the electrical components are enclosed inside a watertightpressure vessel assembly 20 with all points of possible moisture entrysealed at several levels by mechanical and fluid seals.

The pressure vessel assembly, due to the aluminum coating, of thepresent invention is particularly effective in shallow water, or “splashzones”, where the oxygen content of seawater is high. However, sincedeep, oxygen-poor seawater corrodes steel only at a rate of 0.004″ peryear, a vessel made according to the present invention is adequate forlong life applications beyond 20 years.

The pressure vessel and cable termination assembly seals of the presentinvention thus provide a reliable, durable pressure vessel assembly forhousing electronic components in the underwater environment. Moreover,the materials and production methods used result in inexpensive pressurevessels. Specifically, reduced product costs are realized by replacingthe beryllium copper material used for conventional pressure vessels andbulkheads with carbon steel, thereby reducing costs on the order ofabout ten to one. Also, there are no safety hazards in working with thecarbon steel, as opposed to beryllium copper. Epoxy cable sealsdescribed herein are about one fourth the cost of Bridgman-type sealsand installation is more reliable. The use of heat-shrinkable tubing toseal the termination assemblies eliminates the need for costly,specialized capital equipment for polyethylene molding and reducesmanufacturing time. Generally, manufacturing and assembly of thepressure vessel assembly of the present invention are less costly, lesscomplex, require lower skill levels, and are less time-consuming thanpresent pressure vessels. The manufacturing process of the pressurevessel assembly removes process dependent characteristics inherent withpresent pressure vessels by providing a more robust, repeatable product.

Although the present invention has been shown and described inconsiderable detail with respect to only a few exemplary embodimentsthereof, it should be understood by those skilled in the art that we donot intend to limit the invention to the embodiments since variousmodifications, omissions and additions may be made to the disclosedembodiments without materially departing from the novel teachings andadvantages of the invention, particularly in light of the foregoingteachings. For example, the cable termination assembly seal and methodmay be used to seal the passage of any cable into a cable-receivingstructure. Accordingly, we intend to cover all such modifications,omission, additions and equivalents as may be included within the spiritand scope of the invention as defined by the following claims. In theclaims, means-plus-function clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents, but also equivalent structures. Thus, although anail and a screw may not be structural equivalents in that a nailemploys a cylindrical surface to secure wooden parts together, whereas ascrew employs a helical surface, in the environment of fastening woodenparts, a nail and a screw may be equivalent structures.

We claim:
 1. A cable seal for sealing the passage of a cable into acable-receiving structure, the cable including an outer surface, thecable seal comprising: a first plurality of o-rings adapted to bedisposed along a length of the cable adjacent the cable-receivingstructure; a first length of heat-shrinkable tube adapted to fit overthe cable and o-rings adjacent the cable-receiving structure forcompressive engagement with the cable and o-rings when the first tube isheated to prevent moisture penetration between an inner surface of thefirst tube and the outer surface of the cable; a second plurality ofo-rings disposed over the first heat-shrinkable tube, the secondplurality of o-rings positioned adjacent the first plurality of o-rings;and a second length of heat-shrinkable tube adapted to fit over thesecond plurality of o-rings, the first heat-shrinkable tube, the firstplurality of o-rings, the cable and a portion of the cable-receivingstructure for compressive engagement with the second plurality ofo-rings, the first heat-shrinkable tube, the first plurality of o-rings,the cable and the portion of the cable-receiving structure when heatedto prevent moisture penetration between an inner surface of the secondtube and an outer surface of the cable-receiving structure and betweenthe inner surface of the second tube and the outer surface of the cable.2. A cable seal as recited in claim 1, wherein the inner surfaces of thefirst and second tubes are coated with an adhesive.
 3. A sealed cableend assembly, the cable end assembly comprising a cable having aterminal end and an outer surface; a structure having a through passagefor receiving the cable end, the cable-receiving structure includingmeans for preventing relative axial movement of the structure and cable;a first plurality of o-rings adapted to be disposed along a length ofthe cable adjacent the cable-receiving structure; a first length ofheat-shrinkable tube positioned around the cable and o-rings adjacentthe cable-receiving structure for compressive engagement with the cableand o-rings when the first tube is heated to prevent moisturepenetration between an inner surface of the first tube and the outersurface of the cable; a second plurality of o-rings disposed over thefirst heat-shrinkable tube, the second plurality of o-rings positionedadjacent the first plurality of o-rings; and a second length ofheat-shrinkable tube positioned around the second plurality of o-rings,the first heat-shrinkable tube, the first plurality of o-rings, thecable and the portion of the cable-receiving structure for compressiveengagement with the second plurality of o-rings, the firstheat-shrinkable tube, the first plurality of o-rings, the cable and aportion of the structure cable when heated to prevent moisturepenetration between an inner surface of the second tube and an outersurface of the portion of the structure and between a portion of theinner surface of the second tube and the outer surface of the cable. 4.A cable seal as recited in claim 3, wherein the inner surfaces of thefirst and second tubes are coated with an adhesive.
 5. A cable seal asrecited in claim 3, wherein the periphery of the outer surface of theportion of the cable-receiving structure further comprises a raised lipso that the second heat-shrinkable tube compressively engages the lipwhen heated.
 6. A method for sealing a cable termination, the cabletermination including a structure having an opening for receiving acable end and preventing relative axial movement of the cable end andstructure, the cable termination sealing method comprising the steps of:disposing a first plurality of o-rings along a length of the cableadjacent the cable-receiving structure; positioning a first length ofheat-shrinkable tube around the cable and o-rings; heating the firstheat-shrinkable tube causing the tube to compressively engage the cableand o-rings to prevent moisture penetration between an inner surface ofthe first tube and the outer surface of the cable; disposing a secondplurality of o-rings disposed around the first heat-shrinkable tube, thesecond plurality of o-rings positioned adjacent the first plurality ofo-rings; positioning a second length of heat-shrinkable tube around thesecond plurality of o-rings, the first heat-shrinkable tube, the firstplurality of o-rings, the cable and a portion of the cable-receivingstructure; and heating the second heat-shrinkable tube causing the tubeto compressively engage the second plurality of o-rings, the firstheat-shrinkable tube, the first plurality of o-rings, the cable and aportion of the structure to prevent moisture penetration between aninner surface of the second tube and an outer surface of the portion ofthe structure and between a portion of the inner surface of the secondtube and the outer surface of the cable.